Cylindrical rotary strainer

ABSTRACT

A cylindrical strainer equipped with rotary screen cleaning mechanism featuring a scraper structure comprising one or more upright spiral scraper elements provided with squegee strips, the scraper structure being connected loosely to a depending vertical drive shaft, rendering the scraper structure self centering relative to the surrounding screen while providing unobstructed through flow area surrounded by the scraper structure.

United States Patent 1 1 11 3,762,563 Petersen 1 1 Oct. 2, 1973 [541 CYLINDRICAL ROTARY STRAINER 1,483,667 2/1924 Landrcth ct a1. 210/413 10,8 10 1924 R J 2 [76] Inventor: Paul F. Petersen, 1096 E. Mam St., I 5 63 08c X Stamford, Conn.

Primary Examiner.l0hn Adee.

1 Filed: Attorney-Burtsell J. Kearns and Theodore M. .lablon [21] Appl. No.: 118,146

[57] ABSTRACT [52] 11.8. C1. 210/415, 209/273 A cylindrical Strainer equipped with rotary Screen [51] Int. Cl B0ld 29/38 cleaning mechanism featuring a Scraper Structure [58] Field of Search 210/413, 414, 415; prising one or more upright Spiral Scraper elements 209/273 provided with squegee strips, the scraper structure being connected loosely to a depending vertical drive [56] ReerenceS cued shaft, rendering the scraper structure self centering rel- UNITED STATES PATENTS ative to the surrounding screen while providing unob- 2,713,921 7/1955 Turner 210/415 X structed through flow area surrounded by the scraper 2,125,532 8/1938 Wells 210/415 X structure. 2,554,492 5/1951 Hartman et a1... 210/415 X 3,375,776 4/1968 Dyson 210/415 X 15 Claims, 13 Drawing Figures Pmamaunm 2 3.762.563

' SHEET 10F 3 INVENTOR.

PAUL PETERSEN ATTORNEY.

PAIENTEDBBT 2 SHEET 2 [IF 3 INVENTOR.

PAUL PETERSEN ATTORNEY.

Pmmin 'fl 3.762.563

SHEET 30F 3 FIG.

PAUL PETERSEN ATTORNEY.

CYLINDRICAL ROTARY STRAINER This invention relates to cylindrical strainers equipped with rotary cleaning mechanism for continuously removing from the interior of the cylindrical screen element the contaminants or oversize material retained from a suspension or pulp or fluid stream supplied under pressure to the screen.

Important applications for such strainers are for instance in any food, pharmaceutical or chemical plant flow sheet, for use in th straining of the feed to centrifuges and other processing machines, wherein oversize particles may be detrimental to the process and/or to the machine.

More in particular, this invention is concerned with improvements in a type of upright strainer unit wherein the cylindrical screen elment is surrounded by a housing which comprises a feed inlet head portion at the top, for pressure feeding to the interior of the screen element, an intermediate body portion surrounding the screen element, to constitute an annular discharge chamber for the screened liquid or suspension, and a bottom outlet portion discharging collected oversize material or residue in a direction downwardly from the interior of the screen.

A shaft extends-through the head portion of the housing carrying screen cleaning elements or scrapers coaxial with the screen. A motor mounted on the housing may rotate the shaft, whereby the oversize or residue material is removedcontinuously from the screen collecting at the bottom for removal through a downwardly outlet means. Operation of a discharge valve will allow collected material to be discharged under pressure continuously or periodically.

Screen cleaning mechanism heretofore employed in the above type strainer unit was prone to cause clogging due to viscous and fibrous screen residue accumulating upon the spider arms supporting the lower end bearing for the scraper shaft, located in the bottom outlet portion of the housing.

One object is to provide a strainer unit having improved screen cleaning mechanismwhich provides unobstructed throughflow passage from top inlet to bottom outlet of the housing, while effectively performing the screen cleaning operation.

Another object is to provide a cleaning mechanism.

wherein the pressures of the cleaning elements engaging the screen surface are balanced relative to one another. 1

Still other objects are to provide a cleaning mechanism of great simplicity and accessability, and operable with a minimum of wear and tear.

These objects are attainable by providing a screen cleaning mechanism which eliminates the conventional lower end bearing for the shaft. This mechanism comprises a shortened shaft or stub shaft extending through the top end or head portion of the housing substantially coaxial with the screen, a screen engaging scraper structure rotatable coaxial with the screen, and rotation transmitting means loosely connecting said scraper structure with the lower end axial portion of the shaft, while securing said structure against axial displacement. This arrangement allows for self-centering selfadjustment of the scraper structure relative to the screen as well as for unobstructed flow into and through the interior of the screen and through the bottom outlet of the housing.

More specifically, this mechanism may comprise a scraper structure featuring one or more upright spirally shaped scraper members hugging the interior surface of the screen, a transverse member connecting the scrapers rigidly to one another, and rotation transmitting means loosely connecting the transverse member of the scraper structure to the lower end of the shaft. Unobstructed through flow passage is then obtainable through a central area surrounded by the special shaped scraper members.

Specific features are found in the construction of the scaper structure comprising a preferably tubular spiral member having a squegee strip of flexible material removably attached thereto, along the length thereof.

Other features and advantages will hereinafter appear.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiments are illustrative and not restrictive. The scope of the invention is defined by the appended claims rather than by the description preceding them, and all embodiments which fall within the meaning and range of equivalency of the claims are therefore intended to be embraced by those claims.

FIG. 1 is a vertical sectional view of the strainer unit, showing the improved screen cleaning mechanism featuring a floatingly mounted scraper structure surrounding an unobstructed through-flow passage.

FIG. 1a is a detail sectional view taken on line la la in FIG. 1, showing the mounting of a squeegee on a tubular spiral scraper member.

FIG. 2 is a cross-sectional view taken on line 2 2 in FIG. 1. I

FIG. 2a is a cross-sectional view taken on line 2a 2a in FIG. 1, but showing a modified scraper structure havi g three spirally shaped scraper elements.

FIG. 3 shows another form of the scraper structure of this invention.

FIG. 4 is atop view taken on line 4 4 of FIG. 3.

FIG. 5 shows an example of an application of the strainer unit supplying screened feed'liquid or suspension to a centrifugal machine. I

FIG. 6 is vertical sectional view of a-modification of the scraper structure shown in FIG. 1.

FIG. 7 is a vertical sectional view taken on line 7 7v in FIG. 6.

FIG. 8 is a vertical sectional view of a scraper structure similar to FIGS. 6 and'7, slightly modified.

FIG. 9 is a vertical sectional view taken on line 9 9 in FIG. 8.

FIG. 10 is fragmentary view fied.

FIG. 11 is vertical sectional view of a scraper structure similar to FIG. 3, modified.

The improved rotary screen cleaning mechanism as herein exemplified may be substituted in an existing upright strainer unit to provide unobstructed throughflow passage, requiring only to omit the conventional lower end bearing for the shaft, and to provide a loose connection between the upper end of a novel scraper structure and the lower end of a shortened shaft. In this example, two upright spiral-shaped scraper members of the scraper structure are rigidly inter-connected only at their upper ends. Thus, in the absence of the lower bearing support, there is unobstructed through flow passage through the unit, namely through an open area of FIG. 9, further modisurrounded by the scraper members, with discharge of screen residue through the downwardly directed unencumbered bottom outlet means.

Accordingly, in this strainer unit a cylindrical screen member is surrounded by a housing which comprises a head or top end portion 11 communicating with the interior of the screen, and having a feed inlet connection 12, an intermediate or body portion 13 surrounding the screen, and forming a receiving chamber 14 for the screened liquid discharging through a lateral outlet connection 14a, and a bottom portion 15 with downwardly directed outlet means 16 for discharging screen residue through a throttle valve 17.

The housing has a removable top cover member 18 held firmly in place against an upper sealing ring 19. A downward extension 20 of the cover member in turn holds the screen member down in place and through sealing ring 19 in sealed relationship with respect to the surrounding housing portion. A lower sealing ring 21 is effective between the lower end of the screen member and the housing, seal 21 being confined by retainer ring 21a. A guide pin 21b extends from the lower free end of each tubular scraper member, having guide relationship with ring 21a.

A stub shaft 23 extends through the housing concentric with the screen member, surrounded by a stuffing box 24 provided in the cover member. A coupling 25 connects the shaft to a motor drive unit 26. A support structure for the motor comprises a vertical rod 27 held on the housing in vertically spaced lugs 28 and 29. A bracket 29a bolted to the base of the motor unit is slidable as well as turnable upon the top end portion of the rod. A sleeve 29b surrounding the rod, defines the normal operating position of the motor unit 26. Thus, the motor unit may be disconnected from the shaft and scraper structure simply by sliding it upward on'the rod far enough to disconnect the coupling 25, and then swinging the unit about the rod to an out-of-the-way position. This allows the cover member 18 together with the attached rotary scraper mechanism, aswell as the cylindrical screen to be lifted from the housing;

Connected to the'lower end of stub shaft 23 is the upper end of a scraper structure 30 comprising a hub portion 31 loosely connected to the shaft, a pair of spiral shaped scraper members 31a and 31b, and a pair of arms 31c and 31d rigidly connecting the hub member with the upper ends of the spiral shaped scraper members. Thus, the two arms and the hub member together constitute a transverse or tie member rigidly interconnecting the upper ends of the scraper members. This leaves the free lower ends of the scraper members as well as the bottom outlet 16 unencumbered, thus providing free throughflow passage from feed inlet to sludge outlet, thus to allow for unobstructed continuous operation without clogging.

Referring to FIG. 1a, the scraper members comprise a tubular element 31a to which are fastened squeegee stri'ps 31f consisting of a resiliently deflectable material. The squeegee strip is fastened tangentially to the tubular element for instance by means of bolt connections 31g.

The linear spiral length of the scraper members from end to end depends upon the pitch or slope of the spiral shape. In the example of FIG. 2, the linear length corresponds to about one-half of the pitch or nearly 180 as indicated by are A.

The cross-sectional view in FIG. 2a taken in FIG. I is for the purpose of showing a modified scraper structure having three spirally shaped scraper elements E-l, E-2, and E-3 instead of the two scraper elements shown in FIGS. 1 and 2. Each of these three tubular scraper members has a Squegee strip S attached thereto by way of fastening means which may be similar to those exemplified in detail FIG. la.

In the embodiment of FIG. 3, the scraper structure comprises a single tubular member 32 having a spiral linear length which may correspond to nearly a full putch P of the spiral. A downwardly inclined arm 33 provides the rigid connection between the upper end of the scraper member and a hub member 34 loosely connected to stub shaft 23. Squegee action during rotation of the scraper structure is due to the provision of a strip 35 of resiliently flexible strip material which may be fastened to the tubular member 32 in the manner shown above in FIG. 1a. In this embodiment too, the scraper structure is capable of self-centered selfadjustment relative to the screen member because of the loose connection with the stub shaft, and because of contact point P-l of the scraper member balancing with contact points P-2 and P-3.

In FIGS. 6 and 7 a modification of the scraper structure differs from FIG. 1, in that a pair of spiral shaped scraper members 36 and 37 are rigidly interconnected by a horizontal tie member 38 shown to be located intermediate or midway between the ends of the scraper members. Embodied in this tie member there is a hub portion 39 loosely connected to the lower end of an extension rod 40 the upper end of which in turn has a loose connection or coupling 41 with stub shaft 23. Again, this scraper structure is self centering relative to a screen element 42.

In the modification of FIGS. 8 and 9 the scraper structure differs from that in FIGS. 6 and 7, in that a horizontal tie member 43 between the scraper members is slidable as well as turnable in a horizontal sleeve member 44 detachably connected at 45 to the lower end of a vertical extension rod 46 the upper end of which has a loose connection 47 with stub shaft 23 as through a loose sleeve or coupling member 47a.

In the modification of FIG. 10, the detachable connection 45 of FIGS. 8 and 9 is replaced by a rigid welded connection 48 between a horizontal sleeve 49 and a vertical extension rod 50. A horizontal tie. rod 50a is slidable as well as turnable in-sleeve 49.

In the modification of FIG. 11, the scraper structure consisting of a single spiral shaped member 51 differs from the one in FIG. 3 in that the scraper member is connected at one or more levels by horizontal arms or spokes 52, 52a, 52b to a downward extension rod 53 of hub member 53a of scraper member 51.

As illustrated in the example of FIG. 5, the strainer unit 54 representing any of the foregoing embodiments may be applied for conditioning a pulp before feeding it into a centrifugal machine 55, the scale of the strainer unit being greatly exaggerated, relative to the centrifugal machine. Accordingly, a pump 56 delivers the pulp containing undesirable oversize material into the strainer 54 wherein the feed pressure forces liquid carrying more uniformly sized smaller particles, first through the screen, and then through transfer pipe 57 to the feed inlet of the nozzle type centrifuge, the mode of operation of which is well known. The feed pressure in the strainer unit also causes the oversize or residue scraped from the screen, to be sloughed off into receiver 58 either periodically or continuously, by the operation or setting of a discharge valve 59.

From the foregoing it will be seen that the invention provides an unobstructed bottom outlet zone, which avoids clogging, this being achieved by the provision of one or more upright spirally shaped scraper members constituting a scraper structure loosely connected to the drive shaft in such a manner and so constructed as to cooperate with the surrounding screen element in self-centering relationship therewith.

I claim: 1. A pulp strainer unit which comprises a generally cylindrical screen, a housing having an intermediate body portion surrounding said screen in sealed relationshp with the top and bottom ends of the screen, and constituting with said screen an annular receiving chamber provided with outlet means for the screened liquid, a bottom portion providing a collecting space for pulp material intercepted by the screen, and having discharge valve means communicating with the bottom opening of the screen, operable to maintain pulp under pressure, and further having a top portion forming a pulp influent chamber communicating with the top opening of the screen, provided with a pulp feed connection, and further having a top closure member, removably connected to the top of said influent chamber, drive shaft sealingly mounted in said top closure member coaxial with said cylindrical screen, secured against undue axial displacement, at least a pair of spiral-shaped substantially rigid scraper members evenly spaced from one another about the axis of the screen, a transverse member rigidly interconnecting the scraper members so as to constitute therewith a rotary scraper structure surrounded by said screen, each of said spiral members having connected thereto resiliently yieldable strip material presenting resiliently 1 yieldable screen engaging edge portions along said scraper members, for wiping residue from the screen into the receiving space below, when rotating the scraper structure, I

rotation transmitting means between said shaft and said transverse member of the scraper structure, providing a loose connection effective to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby effective to interbalance and equalize the pressures of said screen engaging edge portions relative to the screen,

and means for rotating said scraper structure through said shaft.

2.,The strainer unit according to claim I, wherein said transverse member of the scraper structure comprises a hub portion loosely connected to the lower end portion of saidshaft.

3. The strainer unit according to claim 1, wherein said transverse member is connected to said scraper members at the upper ends thereof, said scraper members thus having free lower ends providing maximum open throughflow area at the lower end of the screen,

4. The strainer unit according to claim 1, wherein said transverse member is connected to said scraper members a substantial distance from the lower ends thereof, said scraper members thus having free lower ends providing maximum open throughflow area at the lower end of the screen.

5. The strainer unit according to claim 1, wherein said transverse member comprises a straight horizontal member interconnecting a pair of scraper members, and wherein said rotation transmitting means comprise a loose fitting sleeve surrounding said horizontal member, having torque transmitting connection with the lower end of said shaft, said sleeve allowing for horizontal sliding movement therein of said transverse member, and thus for self-centering adjustment of the scraper structure.

6. A pulp strainer unit which comprises a generally cylindrical screen,

a housing having an intermediate body portion surrounding said screen in sealed relationship with the top and bottom ends of the screen, and constituting said screen an annular receiving chamber provided with outlet means for the screened liquid, a bottom portion providing a collecting space for pulp material intercepted by the screen, and having discharge valve means communicating with the bottom opening of the screen, operable to maintain pulp under pressure, and further having a top portion forming a pulp influent chamber communicating with the top opening of the screen, provided with a pulp feed connection, and further having a top closure member, removably connected to the top of said influent chamber, drive shaft sealingly mounted in said top closure member coaxial with said cylindrical screen, secured against undue axial displacement,

single spiral-shaped substantially rigidscraper member substantially coaxial with the screen, having at least a single complete winding of a length substantially corresponding to the length of the screen, and having a radial arm extending rigidly inwardly from said scraper member, to constitute therewith a rotary scraper structure surrounded by said screen, coaxial therewith, said spiral member being provided with screen engaging means comprising resiliently yieldable strip material presenting a resiliently yieldable screen engaging edge portion for wiping residue from the screen into the re-v ceiving space below, when rotating the scraper structure, rotation transmitting means between said shaft and said radially extending member of the scraper structure, providing a loose connection effective to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby effective to interbalance and equalize the pressures of said screen engaging edge portion relative to the screen, and means for rotating said scraper structure.

7. The strainer unit according to claim 6, wherein said arm comprises a hub portion loosely connected to the lower end portion of said shaft.

8. The strainer unit according to claim 6, wherein said transverse member is connected to said scraperv members at the upper end thereof, said scraper member thus having a free lower end providing maximum open throughflow area at the lower end of the screen.

9, The strainer unit according to claim 6, wherein said transverse member is connected to said scraper members a substantial distance from the lower end thereof, said scraper members thus having free lower ends providing maximum open throughflow area at the lower end of the screen.

10. In a strainer unit having a generally cylindrical screen mounted in a strainer housing having a top closure,

a screen cleaning mechanism which comprises a shaft adapted to be sealingly mounted in the top closure of said housing coaxial with the screen, while secured against axial displacement in said top closure,

at least a pair of spiral-shaped substantially rigid scraper members evenly spaced from one another about the axis of said shaft, a' transverse member rigidly interconnecting the scraper members so as to constitute therewith a rotary scraper structure adapted to be surrounded by said screen, each of said spiral members having connected thereto resilient yieldable strip material presenting resiliently yieldable edge portions along said scraper members, adapted to wipe residue from the screen into a receiving space below during rotation of the scraper structure,

and rotation transmitting means between said shaft and said transverse member of the scraper structure, providing a loose connection adapted to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to'said shaft,-and thereby adapted to interbalance and equalize the pressures of said yieldable edge portions relative to the screen, said scraper structure being adapted to be rotated through said shaft.

11. In a strainer unit ha ving a generally cylindrical screen mounted in a strainer housing having a top closure,

a screen cleaning mechanism which comprises a shaft adapted to be sealingly mounted in the top closure of said housing coaxial with the screen, while secured against axial displacement insaid top closure,

single spiral-shaped substantially rigid scraper member substantially coaxial with said shaft, having atleast a single complete winding of a length substantially corresponding to the length of the screen, and having a radial arm extending rigidly inwardly from said scraper member, to constitute therewith a rotary scraper structure adapted to be surrounded by said screen, coaxial therewith, said spiral member having connected thereto resiliently yieldable strip material presenting a resiliently yieldable edge portion along said scraper member, adapted to wipe residue from the screen into a receiving space below during rotation of said scraper structure, I and rotation transmitting means between said shaft and said transverse member of the scraper structure, providing a loose connection adapted to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby ada ted to interbalance and equalize the pressures said yieldable edge portions relative to the screen, said scraper structure being adapted to be rotated through said shaft.

12. in a strainer unit having a generally cylindrical screen mounted in a strainer housing having a shaft sealingly mounted in a top closure of the housing, coaxial with the screen, while secured against axial displacement in said top closure,

a screen cleaning structure which comprises at least a pair of spiral-shaped substantially rigid scraper members evenly spaced from one another about an axis, and a transverse member rigidly interconnecting the scraper members so as to constitute therewith a rotary scraper structure adapted to be surrounded by said screen, each of said spiral members having connected thereto resiliently yieldable strip material presenting resiliently yieldable edge portions along said scraper members, adapted to wipe residue from the screen into a receiving space below during rotation of said scraper members substantially coaxial with said shaft, said scraper structure adapted to have its transverse member loosely connected to said shaft in a manner to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby adapted to interbalance and equalize the pressures of said yieldable edge portions relative to the screen, said'scraper structure being adapted to be rotated through said shaft.

13. The screen cleaning structure according to claim 12, wherein said transverse member is formed with a hubportion.

14. In a strainer unit having a generally cylindrical screen mounted in a strainer housing having a shaft sealingly mounted in a top closure of the housing, coaxial with the screen, while secured against axial displace ment in said top closure,

a screen cleaning structure which comprises a single spiral-shaped substantially rigid scraper member substantially coaxial with said shaft, having at least a single complete winding of a length substantially corresponding to the length of the screen, and having a radial arm extending rigidly inwardly from said scraper member, to constitute therewith arotary scraper structure adapted to be surrounded by said screen, coaxial therewith, said spiral member having connected thereto resiliently yieldable strip material presenting a resiliently yieldable edge portion, adapted to wipe residue from the screen into a receiving space below during rotation'of said scraper structure, said scraper structure adapted to have its radial arm loosely connected to said shaft in a manner to allow for self-centering selfadjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby adapted to interbalance and equalize the pressures of said yieldable edge portions relative to the screen, said scraper structure being adapted to be rotated through said shaft. l5. The screen cleaning structure according to claim 14, wherein said radial arm is formed with a hub portion. 

1. A pulp strainer unit which comprises a generally cylindrical screen, a housing having an intermediate body portion surrounding said screen in sealed relationshp with the top and bottom ends of the screen, and constituting with said screen an annular receiving chamber provided with outlet means for the screened liquid, a bottom portion providing a collecting space for pulp material intercepted by the screen, and having discharge valve means communicating with the bottom opening of the screen, operable to maintain pulp under pressure, and further having a top portion forming a pulp influent chamber communicating with the top opening of the screen, provided with a pulp feed connection, and further having a top closure member, removably connected to the top of said influent chamber, a drive shaft sealingly mountEd in said top closure member coaxial with said cylindrical screen, secured against undue axial displacement, at least a pair of spiral-shaped substantially rigid scraper members evenly spaced from one another about the axis of the screen, a transverse member rigidly interconnecting the scraper members so as to constitute therewith a rotary scraper structure surrounded by said screen, each of said spiral members having connected thereto resiliently yieldable strip material presenting resiliently yieldable screen engaging edge portions along said scraper members, for wiping residue from the screen into the receiving space below, when rotating the scraper structure, rotation transmitting means between said shaft and said transverse member of the scraper structure, providing a loose connection effective to allow for self-centering selfadjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby effective to interbalance and equalize the pressures of said screen engaging edge portions relative to the screen, and means for rotating said scraper structure through said shaft.
 2. The strainer unit according to claim 1, wherein said transverse member of the scraper structure comprises a hub portion loosely connected to the lower end portion of said shaft.
 3. The strainer unit according to claim 1, wherein said transverse member is connected to said scraper members at the upper ends thereof, said scraper members thus having free lower ends providing maximum open throughflow area at the lower end of the screen.
 4. The strainer unit according to claim 1, wherein said transverse member is connected to said scraper members a substantial distance from the lower ends thereof, said scraper members thus having free lower ends providing maximum open throughflow area at the lower end of the screen.
 5. The strainer unit according to claim 1, wherein said transverse member comprises a straight horizontal member interconnecting a pair of scraper members, and wherein said rotation transmitting means comprise a loose fitting sleeve surrounding said horizontal member, having torque transmitting connection with the lower end of said shaft, said sleeve allowing for horizontal sliding movement therein of said transverse member, and thus for self-centering adjustment of the scraper structure.
 6. A pulp strainer unit which comprises a generally cylindrical screen, a housing having an intermediate body portion surrounding said screen in sealed relationship with the top and bottom ends of the screen, and constituting said screen an annular receiving chamber provided with outlet means for the screened liquid, a bottom portion providing a collecting space for pulp material intercepted by the screen, and having discharge valve means communicating with the bottom opening of the screen, operable to maintain pulp under pressure, and further having a top portion forming a pulp influent chamber communicating with the top opening of the screen, provided with a pulp feed connection, and further having a top closure member, removably connected to the top of said influent chamber, a drive shaft sealingly mounted in said top closure member coaxial with said cylindrical screen, secured against undue axial displacement, a single spiral-shaped substantially rigid scraper member substantially coaxial with the screen, having at least a single complete winding of a length substantially corresponding to the length of the screen, and having a radial arm extending rigidly inwardly from said scraper member, to constitute therewith a rotary scraper structure surrounded by said screen, coaxial therewith, said spiral member being provided with screen engaging means comprising resiliently yieldable strip material presenting a resiliently yieldable screen engaging edge portion for wiping residue from the screen into the receiving space below, when rotating the scraper structure, rotation transMitting means between said shaft and said radially extending member of the scraper structure, providing a loose connection effective to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby effective to interbalance and equalize the pressures of said screen engaging edge portion relative to the screen, and means for rotating said scraper structure.
 7. The strainer unit according to claim 6, wherein said arm comprises a hub portion loosely connected to the lower end portion of said shaft.
 8. The strainer unit according to claim 6, wherein said transverse member is connected to said scraper members at the upper end thereof, said scraper member thus having a free lower end providing maximum open throughflow area at the lower end of the screen.
 9. The strainer unit according to claim 6, wherein said transverse member is connected to said scraper members a substantial distance from the lower end thereof, said scraper members thus having free lower ends providing maximum open throughflow area at the lower end of the screen.
 10. In a strainer unit having a generally cylindrical screen mounted in a strainer housing having a top closure, a screen cleaning mechanism which comprises a shaft adapted to be sealingly mounted in the top closure of said housing coaxial with the screen, while secured against axial displacement in said top closure, at least a pair of spiral-shaped substantially rigid scraper members evenly spaced from one another about the axis of said shaft, a transverse member rigidly interconnecting the scraper members so as to constitute therewith a rotary scraper structure adapted to be surrounded by said screen, each of said spiral members having connected thereto resilient yieldable strip material presenting resiliently yieldable edge portions along said scraper members, adapted to wipe residue from the screen into a receiving space below during rotation of the scraper structure, and rotation transmitting means between said shaft and said transverse member of the scraper structure, providing a loose connection adapted to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby adapted to interbalance and equalize the pressures of said yieldable edge portions relative to the screen, said scraper structure being adapted to be rotated through said shaft.
 11. In a strainer unit having a generally cylindrical screen mounted in a strainer housing having a top closure, a screen cleaning mechanism which comprises a shaft adapted to be sealingly mounted in the top closure of said housing coaxial with the screen, while secured against axial displacement in said top closure, a single spiral-shaped substantially rigid scraper member substantially coaxial with said shaft, having at least a single complete winding of a length substantially corresponding to the length of the screen, and having a radial arm extending rigidly inwardly from said scraper member, to constitute therewith a rotary scraper structure adapted to be surrounded by said screen, coaxial therewith, said spiral member having connected thereto resiliently yieldable strip material presenting a resiliently yieldable edge portion along said scraper member, adapted to wipe residue from the screen into a receiving space below during rotation of said scraper structure, and rotation transmitting means between said shaft and said transverse member of the scraper structure, providing a loose connection adapted to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby adapted to interbalance and equalize the pressures of said yieldable edge portions relative to the screen, said scraper structure beinG adapted to be rotated through said shaft.
 12. In a strainer unit having a generally cylindrical screen mounted in a strainer housing having a shaft sealingly mounted in a top closure of the housing, coaxial with the screen, while secured against axial displacement in said top closure, a screen cleaning structure which comprises at least a pair of spiral-shaped substantially rigid scraper members evenly spaced from one another about an axis, and a transverse member rigidly interconnecting the scraper members so as to constitute therewith a rotary scraper structure adapted to be surrounded by said screen, each of said spiral members having connected thereto resiliently yieldable strip material presenting resiliently yieldable edge portions along said scraper members, adapted to wipe residue from the screen into a receiving space below during rotation of said scraper members substantially coaxial with said shaft, said scraper structure adapted to have its transverse member loosely connected to said shaft in a manner to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby adapted to interbalance and equalize the pressures of said yieldable edge portions relative to the screen, said scraper structure being adapted to be rotated through said shaft.
 13. The screen cleaning structure according to claim 12, wherein said transverse member is formed with a hub portion.
 14. In a strainer unit having a generally cylindrical screen mounted in a strainer housing having a shaft sealingly mounted in a top closure of the housing, coaxial with the screen, while secured against axial displacement in said top closure, a screen cleaning structure which comprises a single spiral-shaped substantially rigid scraper member substantially coaxial with said shaft, having at least a single complete winding of a length substantially corresponding to the length of the screen, and having a radial arm extending rigidly inwardly from said scraper member, to constitute therewith a rotary scraper structure adapted to be surrounded by said screen, coaxial therewith, said spiral member having connected thereto resiliently yieldable strip material presenting a resiliently yieldable edge portion, adapted to wipe residue from the screen into a receiving space below during rotation of said scraper structure, said scraper structure adapted to have its radial arm loosely connected to said shaft in a manner to allow for self-centering self-adjustment of said scraper structure relative to the surrounding screen, while secured against undue axial displacement relative to said shaft, and thereby adapted to interbalance and equalize the pressures of said yieldable edge portions relative to the screen, said scraper structure being adapted to be rotated through said shaft.
 15. The screen cleaning structure according to claim 14, wherein said radial arm is formed with a hub portion. 